1. Field of the Invention
The present invention relates to a non-aqueous electrolyte solution for a lithium secondary battery, which includes a dinitrile compound in which a hetero atom is substituted at a main chain to prevent a swelling phenomenon of the battery and a lithium secondary battery including the same.
2. Description of the Related Art
Recently, attention on an energy storing technique is increasing more and more. As the application field of the energy storing technique is enlarged to a cellular phone, a camcorder and a notebook personal computer (PC), and by extension, to an electric vehicle, requirements on a battery having a high energy density, which is used as the power supply of an energy electronic device, are increasing. A lithium secondary battery is the most appropriate battery satisfying the above-described requirements, and researches on the lithium secondary battery are actively conducted nowadays.
Among recently applied secondary batteries, a lithium secondary battery developed in the early 1990s includes an anode formed by using a carbon material etc. for absorbing and releasing lithium ions, a cathode formed by using an oxide including lithium, etc. and a non-aqueous electrolyte solution including an appropriate amount of a lithium salt dissolved in a mixed organic solvent.
The organic solvent widely and presently used in the non-aqueous electrolyte solution includes ethylene carbonate, propylene carbonate, dimethoxyethane, gamma butyrolactone, N,N-dimethylformamide, tetrahydrofuran, acetonitrile, etc. However, the above described solvents may generate a gas due to the oxidation of an electrolyte when stored at a high temperature for a long time. In this case, the structure of the battery may be deformed, or an internal short may be generated because of an internal heating due to an overcharge or an over-discharge to induce the ignition or the explosion of the battery.
Recently, in order to solve the above-described limitations, methods for improving the stability of the battery at a high temperature by (1) using a porous polyolefin-based separator having a high melting point and hardly melting at a high temperature surroundings or (2) mixing a flame-retardant solvent or an additive with an electrolyte, have been attempted.
However, the thickness of the porous polyolefin-based separator is commonly required to be increased to accomplish the high melting property. Accordingly, the loading amounts of the anode and the cathode relatively decrease, and the decrease of the capacity of the battery becomes inevitable. In addition, since the melting point of the polyolefin-based separator formed by using PE, PP is about 150° C., the separator may be molten due to the rapid internal heating caused by the oxidation of the electrolyte during over-charging. In this case, an internal short of the battery may be induced and the ignition and the explosion of the battery may be inevitable.
Recently, various researches on developing an electrolyte having new components including an additive have been conducted to solve the above-described limitations. For example, a nonflammable gas having a boiling point of 25° C. or less may be added, a phosphoric acid ester may be added into a carbonate to confirm the nonflammability of the electrolyte, or 30% or more of a nonflammable solute of a perfluoroalkyl or a perfluoro ester may be added into the electrolyte. However, when the nonflammable gas is injected into the electrolyte, the volume of the battery may increase, and a complicated electric assembling process maybe required to be conducted. In addition, when the phosphoric acid ester is added into the electrolyte, the performance of the battery may be deteriorated due to a high reduction potential. When the perfluoro compound is added into the electrolyte, a lithium salt may be precipitated from the organic solvent electrolyte.
In order to improve the above-described limitations, researches on an electrolyte including an amide compound, which exhibits a wide electrochemical window and a high thermal and chemical stability and solves the limitation on the evaporation, the ignition of the electrolyte due to the use of the common organic solvent, have been accelerated.